We went through the loop on a special post-conference tour in 1983, including through the underpass bridge. We also looked at the site of the tunnel which then was well underway. There were diagrams in the 1983 PWI book but I can't track down a copy. Yes, definitely not a triangle. Coming up the hill to a cross, you would turn left to a level siding, then reverse under the line to the hill face with a siding long enough so you ween't starting on the grade, Similarly standing aside on the downhill, turn left, back under the main and stand level before heading down the hill.